Are you using acid dyes? And are you dyeing wool? Or silk? There are really two different answers to your question: if you are dyeing wool, it is important not to let the temperature change too quickly, or else the wool can felt. That is why it is best to let the dye bath cool to warm, with the wool in it, before removing the fiber and rinsing in warm to cold water. (If you pull the wool out of the hot dye and put in cold water, you might run into problems.) The other answer addresses a different issue: if your dye bath cools too much, your dye can precipitate, and if this happens while you have fabric in the bath, it can get ugly. I usually pull my silk out of the hot dye bath, without letting it cool much at all... So basically, it depends on what you are dyeing... Wool does not like dramatic temperature changes, and acid dyes can precipitate if the dye bath cools too much or too rapidly... I hope this helps!